1. Field of the Invention
The present invention relates to a radio communication system, a radio communication method, a wireless network forming device, a radio transmission unit, a radio transmission method, a radio reception unit and a radio reception method for intercommunicating among radio stations. In particular, the invention relates to a radio communication system, a radio communication method, a wireless network forming device, a radio transmission unit, a radio transmission method, a radio reception unit and a radio reception method for communicating asynchronously by radio without providing any device as a control station.
More specifically, the present invention relates to a radio communication system, a radio communication method, a wireless network forming device, a radio transmission unit, a radio transmission method, a radio reception unit and a radio reception method for communicating asynchronously in a radio communication environment where the carrier sense for determining how a transmission channel is used is difficult. In particular, the invention relates to a radio communication system, a radio communication method, a wireless network forming device, a radio transmission units, a radio transmission method, a radio reception unit and a radio reception method for communicating asynchronously, with a radio transmission channel used efficiently, on the UWB (ultra wide band) system or another radio communication system on which carrier sense is difficult.
2. Description of the Related Art
Improved computer functions have promoted the interconnection of computers for the establishment of LANs (local area networks). A LAN makes it possible to share information such as a file or data and a peripheral device such as a printer. A LAN also makes it possible to transfer electronic mail and data, and to interchange other information.
The computers in the conventional LANs are interconnected by optical fiber cables, coaxial cables or twisted pair cables. The wired LANs require a wiring work, which makes it difficult to build them, and the cables are complicated. After the LANs are built, they are inconvenient because the cable lengths limit the range where the interconnected devices can be moved.
Radio LANs receive attention as systems that relieve users of the cables of the wired LANs. A wireless LAN can eliminate most of the cables in a working space such as an office, making it relatively easy to move the communication terminals such as personal computers (PCs) in this LAN.
In recent years, as wireless LAN systems have become higher in speed and cheaper, they have grown considerably in demand. In particular, the introduction of personal area networks (PANs) has recently been considered for information communication in a small-scale wireless network built among electronic devices existing around.
It is considered suitable or reasonable that a small-scale wireless network formed of comparatively few clients located near each other is built as an ad hoc communication network, through which any terminals can perform asynchronous radio communication directly among them without using a specific access point.
In general, the asynchronous communication in such small-scale wireless networks involves access control on the CSMA/CA (carrier sense multiple access with collision avoidance) system.
The CSMA/CD system is a collision detection system, on which a communication device transmits a signal and receives this signal to determine whether or not its information transmission collides with the information transmission from other communication devices. The CSMA/CD system is used mainly for wired communication. For radio communication, it is difficult for a communication device to receive the signal transmitted by itself. Accordingly, on the CSMA/CA system, a communication device avoids collision by starting to transmit information after checking that no other communication device is transmitting information. Such an access control system makes two or more communication devices capable of multiple access through the same radio transmission channel shared by them.
In general, a method for sensing a carrier on a radio transmission channel involves a mechanism that uses a specific frequency carrier.
In a radio communication system, the terminals forming a wireless network communicate asynchronously by radio. In order for one of the terminals to successfully receive a radio signal transmitted from another, a method for transmitting information with a preamble signal added ahead of it is widely used. According to this method, one of the terminals transmits a preamble signal, which is received by another for synchronism between the two terminals.
Recently, high speed radio transmission technologies have been studied and developed, one of which is the UWB (ultra wide band) system. The UWB system is a radio communication system that achieves high speed data communication by transmitting and receiving data spread over a very wide frequency band on the order of some gigahertz. This frequency band is an ultra high frequency band, which may range from 2 GHz to 6 GHz.
In the UWB system, trains of impulse signals of information are transmitted and received, each having a very short period on the order of hundreds of picoseconds. The occupied bandwidth is such a bandwidth on the order of some gigahertz that the occupied bandwidth divided by the center frequency is about one. The center frequency may range from 1 GHz to 10 GHz. This bandwidth is extremely wide even as compared with the bandwidth used commonly in wireless LANs on the W-CDMA, cdma 2000, SS (spread spectrum) or OFDM (orthogonal frequency division multiplexing) system.
The trains of impulse signals used in the UWB radio communication system have no specific frequency carriers. In this system, carrier sense is difficult because no specific frequency carrier includes a signal component.
Accordingly, the CSMA/CA system used widely in conventional radio communication systems cannot be applied as it is for UWB radio communication.
In order to receive a signal transmitted asynchronously by radio using the UWB radio communication technology, it is necessary to perform detection for a very long time to find whether the signal is present or absent.
In the UWB radio communication system, there is no carrier as exists in the conventional radio communication systems. Therefore, in this system, in order for a terminal as a receiver to detect a signal for initial synchronization of an information signal, it is necessary for a preamble signal more redundant than the conventional one to be added ahead of the transmitted information.